Package contains one dozen (12) washers.
These leather washers help to keep the string knot from marking an indentation on the underside of a baroque tailpiece, or, when used with a modern tailpiece, they help keep the string knot from pulling through the string slot.
The washers with the 3.80mm outside diameter are intended for smaller instruments such as violins, treble viols and so on. The washers with the 5.00mm diameter are designed for larges instruments like the cello and bass viol. Of course, you are welcome to use any size on any instrument.
When choosing the hole size, select the one that is the same as the diameter of the string you want to use it on, or a little larger than the intended string.